1. Chromosomes, Mitosis & Meiosis
Chapter 9

2. Chromosomes Located in the nucleus of eukaryotic cells
Visible only when the cell is dividing
Made of chromatin – DNA + proteins
Chromatin is just the chromosomes in an extended, partially unraveled state
Chromosomes contain genes
Humans have less than 30,000
Genes provide information on cellular function
Chromosomes package a lot of information into a small space

3. Packaging of Chromosomes
Histones – help to coil the DNA molecule
Histones have a slight + charge
DNA has a slight – charge
This attraction forms a clump of 146 base pairs of DNA wrapped around 8 histones – a nucleosome
Nucleosomes help prevent tangling of the DNA
Scaffolding proteins – nonhistone proteins that bundle the nucleosomes and help maintain the chromosome structure

4. Nucleosomes

5. During cell division
Chromosomes consist of identical halves called sister chromatids
Sister chromatids are held together at an area called the centromere
The centromere has a protein disk called a kinetochore

6. The Cell Cycle – the life cycle of a cell
Interphase
Between one cell division and the next
The majority of a cell’s life
When the cell is doing its work
Consists of three phases:
G1 phase  S phase  G2 phase
M phase
Division of the cell
Consists of two phases:
Mitosis (prophase, metaphase, anaphase, telophase)
Cytokinesis

7. The Cell Cycle

8. Interphase – G1 Phase G = gap (no DNA synthesis)
Growth and normal metabolism occurs
Cells that are no longer actively dividing (nerve, skeletal muscle, red blood cells) are arrested in this stage
These cells are said to be in the G0 phase
At the end of this phase necessary enzymes are synthesized

9. Interphase – S phase Synthesis
DNA replicates and histone is synthesized
This is when the sister chromatids are produced

10. Interphase – G2 phase
Cell is entering final preparations for cell division

11. M phase – Mitosis: Prophase
Nuclear envelope breaks down
Nucleolus disappears
Chromosomes (made of two identical sister chromatids) appear as chromatin condenses
Spindle fibers form between centrioles which are now at the poles of the cell

12. Prophase whitefish cell mitosis
Prophase whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited

13. M phase – Mitosis: Metaphase
Spindle fibers attach to the kinetochores of the chromosomes (located at the centromere of the two sister chromatids)
Chromosomes line up along the equatorial plane of the cell, midway between the two poles

14. Metaphase whitefish cell mitosis
Metaphase whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited

15. M phase – Mitosis: Anaphase
Sister chromatids separate at the centromeres
Each chromosome is pulled toward the opposite poles

16. Anaphase whitefish cell mitosis
Anaphase whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited

17. M phase – Mitosis: Telophase
Chromosomes arrive at the poles (now each is a single sister chromatid)
Nuclear envelope begins to re-form around each set of chromosomes
Cytokinesis begins

18. Telophase whitefish cell mitosis
Telophase whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited

19. M phase – Cytokinesis The division of the cytoplasm
Two new cells are called daughter cells
In animal cells:
A cleavage furrow pinches in around the midline of the cell and divides the two cells
In plant cells:
A cell plate forms in the midline and grows across to divide the two cells

20. Daughter Cells whitefish cell mitosis
Daughter Cells whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited

21. Regulation of the cell cycle
Different types of cells have different time frames for the cell cycle
Cell cycle checkpoints help regulate the cell cycle
Some chemicals can affect the cell cycle:
Drugs that stop the cell cycle are used in cancer treatment
These can cause nausea and hair loss side-effects
Hormones may stimulate mitosis
Cytokinins in plants
Certain steroids in animals

22. Name that stage… Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited

23. Name that stage… Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited

24. Name that stage… Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited

25. Name that stage… Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited

26. Name that stage… Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited

27. Name that stage… Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited

28. Confocal image of dividing cell
Confocal image of dividing cell. The markers are mitotic spindle (red), chromosomes (blue) , and peroxisomes (green).
Credit: © Carolina Biological/Visuals Unlimited